Shaykh Ibn Uthaymeen was asked,
“What is the ruling on establishing a second congregational prayer in the masjid knowing that the first congregation did not finish? Also, is their prayer considered nullified?”
“First, if you come while the Imam is in the last tashahud and you are with him in the congregation, do not start [a second] prayer until you complete the prayer so that two congregations do not meet in one place. However, if they do that and they are far from the first congregation and do not confuse them, then there is no harm in this.
We move from this issue to another issue and it is: if we come to a masjid and we did not pray salatul Isha and we find them praying salatul Taraweeh, then we enter upon them in the salatul Taraweeh with the intention of Isha? Next if we are travelers, then we make tasleem with the Imam if we are praying two rakaat. If we are residents we complete what remains of Salatul Isha and we do not establish another congregation for salatul Isha because it is not necessary to have two congregations in one masjid for verily this is a typical example of division.
Even the Messenger (ﷺ) made tasleem one day and found two men, isolated because they prayed in their tents. He said,
ما منعكما أن تصليا؟ قالا: يا رسول الله صلينا في رحالنا، قال لا تفعلا، إذا صليتما في رحالكم، ثم أتيتما مسجد جماعة فصليا معهم
‘”What prevented you from praying?” They said, “Oh Messenger of Allah, we prayed in our tents.” He said, ‘do not do so. If you pray in your tents, then you come to a masjid [in which people are praying] a congregational prayer, then pray with them'”
So he ordered them to pray with the people even if they prayed in their tents so as not to cause division. Also he said,
إنها لكما نافلة
‘It is for you a voluntary prayer’
” End translation. [Reference]
Faisal Ibn Abdul Qaadir Ibn Hassan